Each Azure region includes a set number of fault
domains. Fault domains consist of a group of virtual machines that share a
common power source and network switch. For regions that have at least three
fault domains (3FD), Atlas deploys clusters across three fault domains.
For regions that only have two fault domains (2FD), Atlas deploys clusters
across two fault domains.

The Atlas Add New Cluster form marks regions
that support 3FD clusters as
Recommended, as they provide higher availability.

The number of fault domains in a region has no affect on the number of MongoDB
nodes Atlas can deploy. MongoDB Atlas clusters are always made of
replica sets with a minimum of three MongoDB nodes.

If the selected Azure region has at least three
fault domains, Atlas clusters are split across three fault domains. For
example, a three node replica set cluster would have one node deployed onto
each zone.

3FD clusters have higher availability
compared to 2FD clusters. However, not all
regions support 3FD clusters.

If the selected Azure region has two fault domains,
Atlas clusters are split across the two fault domains. For example, a
three node replica set cluster would have two nodes deployed to one zone and
the remaining node deployed to the other zone.

2FD clusters have a higher chance of loss of
availability in the event of the loss of an zone than 3FD clusters. However, where latency or location are a
priority, a region that supports 2FD clusters
may be preferred.